Broadband Optical Isolator or Circular Polarizer

ABSTRACT

An optical device can be used for circularly polarizing light and/or removing reflected light from an optical system (optical isolation). The optical device can have a polarizing-cube including a pair of prisms and a polarizer. Each prism can have two ends linked by an inner face and two outer faces. The prisms can be attached together at the inner face of each with the polarizer sandwiched between the prisms. Fresnel rhomb(s) can be attached to outer face(s) of the prisms.

CLAIM OF PRIORITY

This is a divisional of U.S. patent application Ser. No. 15/404,435, filed on Jan. 12, 2017, which claims priority to U.S. Provisional Patent Application No. 62/299,229, filed on Feb. 24, 2016, which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present application is related generally to optical devices, particularly optical isolators and circular polarizers.

BACKGROUND

Circularly-polarized light is used in various fields, including imaging/photography, microscopy, biology, data storage, astronomy, communication, and quality inspection. A circular polarizer can be used to create circularly-polarized light. Circular polarizers can be made by thin film technologies. A disadvantage of thin film circular polarizers is a narrow wavelength band of effective performance (e.g. typically <200 nm bandwidth). Circular polarizers can be expensive and complicated to manufacture. It would be beneficial to provide a circular polarizer capable of effectively polarizing a large bandwidth of light, and that could be manufactured relatively simply and inexpensively.

In some optical systems, light can reflect off of optical components. This reflected light can cause problems in some optical systems, such as for example lasers and image projectors. An optical isolator can be used to remove this reflected light from the optical system. Present optical isolators are usually manufactured using thin films, can have a narrow wavelength band of effective performance, can have a low laser-damage threshold, and can be expensive. It would be beneficial to provide a low-cost, high laser-damage threshold, broadband optical isolator with improved ability to withstand laser light.

SUMMARY

It has been recognized that it would be advantageous to provide a circular polarizer capable of effectively polarizing a large bandwidth of light and that could be manufactured relatively simply and inexpensively. It has been recognized that it would be advantageous to provide a low-cost, broadband optical isolator, with high laser-damage threshold. The present invention is directed to various embodiments of optical devices, including circular polarizers and optical isolators, and methods of removing reflected light from an optical system (optical isolation), that satisfy these needs. Each embodiment may satisfy one, some, or all of these needs.

The optical device can comprise a polarizing-cube including a pair of prisms and a polarizer. Each prism can have two ends linked by an inner face and two outer faces. The prisms can be attached together at the inner face of each with the polarizer sandwiched between the prisms. One outer-face of the first prism can define an entry-face and the other outer-face of the first prism can define a reflection-face. One outer-face of the second prism, located opposite of the entry-face of the first prism, can define an exit-face, and the other outer-face of the second prism, located opposite of the reflection-face of the first prism can define an elimination-face. One or more Fresnel rhombs can be attached to an outer face of the prism(s). In one embodiment, the entry-face is not parallel to the exit-face.

A method of removing reflected light from an optical system can comprise (1) emitting light from a light source into an optical isolator, similar to the optical device described above; (2) passing light from the light source, having a first-polarization, through the polarizer and into a Fresnel rhomb (exit-rhomb); (3) circularly polarizing the light in the exit-rhomb and emitting the circularly-polarized light from the distal-end of the exit-rhomb into an optical system; (4) reflecting at least a portion of the circularly-polarized light from the optical system back into the exit rhomb; (5) linearly polarizing, in the exit-rhomb, the circularly-polarized light reflected off of the optical system, to a second-polarization, the second-polarization being opposite of the first-polarization; (6) emitting the light with the second-polarization from the exit-rhomb through the exit-face of the second prism into the second prism, then to the polarizer; and (7) reflecting the light with the second-polarization from the exit-rhomb off of the polarizer to the elimination-face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of two prisms 15 and 16 for use in an optical device, in accordance with an embodiment of the present invention.

FIG. 2 is a schematic perspective view of an optical device 20, including a polarizer 24 sandwiched between two prisms 15 and 16, and three Fresnel rhombs 23 attached to the prisms 15 and 16, in accordance with an embodiment of the present invention.

FIG. 3 is a schematic top view of the optical device 20 of FIG. 2, illustrating light 34 passing through the optical device 20, to circularly-polarize the light 34 and/or to provide optical isolation, in accordance with an embodiment of the present invention.

FIGS. 4-6 are a schematic perspective views of an optical device 40, similar to optical device 20, except that optical device 40 includes a single Fresnel rhomb 23 and shows dimensions (length L, width W_(adj), and width W_(ac)) angles (acute-angle A_(c), adjacent-angle A_(adj), and angle-of-rotation A_(r) ) of the Fresnel rhomb 23, in accordance with an embodiment of the present invention.

FIG. 7 is a schematic side view of an interface between first and second mediums, and shows light transmitted from first medium (glass) into second medium (air), with n₁>n₂ and θ_(i)<θ_(c), where n₁ is an index of refraction of the first medium, n₂ is an index of refraction of the second medium, θ_(i)=the Angle of Incidence of the light, and θ_(c) is the critical angle.

FIG. 8 is a schematic side view of an interface between first and second mediums, and is similar to FIG. 7, except that θ_(i)=θ_(c).

FIG. 9 is a schematic side view of an interface between first and second mediums, and is similar to FIG. 7, except that θ_(i)>θ_(c).

FIG. 10 is a graph of phase shift difference versus angle of incidence for N-BK7 glass during total internal reflection (TIR).

FIG. 11 is a schematic perspective view of an optical device 100, including a polarizer 24 sandwiched between two prisms 15 and 16, and a Fresnel rhomb 23 attached to one of the prisms 16, in accordance with an embodiment of the present invention.

DEFINITIONS

As used herein, the term “cube” means an approximately cube-shaped optical device, usually with six sides. Opposite sides are usually, but not necessarily parallel. Corners are usually, but not necessarily perpendicular. The sides do not have to have the same area with respect to each other. Examples of cubes are shown in the figures (prisms 15 and 16 sandwiched together with polarizer 24 therebetween).

DETAILED DESCRIPTION

Illustrated in FIG. 1 are two prisms, including a first prism 15 and a second prism 16, for use in optical device 20 (see FIGS. 2-3), optical device 40 (see FIGS. 4-6), or optical device 100 (see FIG. 11). Each prism 15 and 16 can have two ends 11 linked by an inner face 12 and two outer faces 13. The ends 11 can be linked by additional faces (see for example prism 16 in FIG. 11); thus, the term “two outer faces” does not restrict the prism 15 or 16 from having more than two outer faces. The ends 11 can be triangular-shaped, or can be another shape as shown in prism 16 in FIG. 11. The inner face 12 and the two outer faces 13 of the prisms 15 and 16 can be rectangular-shaped, square-shaped, or can have a different shape.

One outer-face 13 of the first prism 15 is called an entry-face 12 _(entry) and the other outer-face 13 of the first prism 15 is called a reflection-face 13 _(refl). One outer-face 13 of the second prism 16, which can be located opposite of the entry-face 13 _(entry) of the first prism 15 when assembled into an optical device 20, 40, or 100, is called an exit-face 13 _(exit), and the other outer-face 13 of the second prism 16, which can be located opposite of the reflection-face 13 _(refl) of the first prism 15 when assembled into an optical device 20, 40, or 100, is called an elimination-face 13 _(elim).

Shown in FIGS. 2-3 are two views of an optical device 20, shown in FIGS. 4-6 are three views of an optical device 40, and shown in FIG. 11 is one view of an optical device 100, each of which can be used as a circular polarizer or as an optical isolator. These optical devices 20, 40, and 100 can be broadband (e.g. can effectively polarize a wavelength range of >200 nm in one aspect, >300 nm in another aspect, >400 nm in another aspect, or about 400-1500 nm in another aspect). Because these optical devices 20, 40, and 100 can be made from durable glass prisms and durable wire grid polarizers, they can have a high laser-damage threshold. Because of the simplicity of these optical devices 20, 40, and 100, they can be relatively inexpensive.

The first prism 15 and the second prism 16 can be attached together at the inner face 12 of each with a polarizer 24 sandwiched between the prisms 15 and 16, forming a polarizing-cube. One, two, three, or more Fresnel rhombs 23 can be each attached to an outer face 13 of one of the prisms 15 or 16. “Attached” means (1) the prism 15 or 16 and the Fresnel rhomb 23 are integrally-formed together; or (2) the prism 15 or 16 and the Fresnel rhomb 23 were manufactured separately, then attached by some method such as adhesive or fusion. Each Fresnel rhomb 23 can be attached to a different outer face 13 than the other. If there are only two Fresnel rhombs 23, each can be attached to the same prism 15 or 16, but typically each will be attached to a different prism 15 or 16 than the other.

One of the Fresnel rhombs 23, called an exit-rhomb 23 _(exit), can have a proximal-end 23 _(exitP) attached to the exit-face 13 _(exit) of the second prism 16 and can have a distal-end 23 _(exitD) spaced apart from the exit-face 13 _(exit) and opposite of the proximal-end 23 _(exitP). Another of the Fresnel rhombs 23, called a reflection-rhomb 23 _(refl), can have a proximal-end 23 _(reflP) attached to the reflection-face 13 _(refl) of the first prism 16 and can have a distal-end 23 _(reflD) spaced apart from the reflection-face 13 _(refl) and opposite of the proximal-end 23 _(reflP). Another of the Fresnel rhombs 23, called an elimination-rhomb 23 _(elim), can have a proximal-end 23 _(elimP) attached to the elimination-face 13 _(elim) of the second prism 16 and can have a distal-end 23 _(elimD) spaced apart from the elimination-face 13 _(elim) and opposite of the proximal-end 23 _(elimP).

The Fresnel rhomb(s) 23 can be attached to the prism(s) 15 or 16 by an adhesive. Adhesives can adversely affect the optical properties of transmitted light, so integrally-forming each prism 15 or 16 with its associated Fresnel rhomb(s) 23, or use of an adhesive-free, optical-bonding method may be preferred. For example, see U.S. Pat. No. 6,548,176 and U.S. Pat. No. 6,284,085, both of which are incorporated herein by reference.

Optical properties of transmitted light can be improved if the Fresnel rhomb(s) 23 and the prisms 15 and 16 are made of the same materials. The prisms 15 and 16 and the Fresnel rhomb(s) 23 can be transparent in a light wavelength range of intended use (e.g. infrared light, visible light, ultraviolet light).

An antireflective coating can be applied to faces of the prisms 15 and 16 and faces of the Fresnel rhomb(s) 23 to reduce reflection of entering light. An absorptive coating can be applied to some faces of the prisms 15 and 16 and faces of the Fresnel rhomb(s) 23 to absorb light and prevent or minimize it from exiting through such faces.

Methods of Using

Methods of using optical device 20 are illustrated in FIG. 3. A light source 37 can emit light 34 into the entry-face 13 _(entry) of the first prism 15. Light with a first-polarization 34 _(p) (e.g. p-polarized light) can pass through the polarizer 24 and into the proximal-end 23 _(exitP) of the exit-rhomb 23 _(exit) where it can be circularly-polarized. The light 34 can be broadband, and the optical device 20 can provide a broad bandwidth of circularly-polarized light 34 _(c), such as for example at least 400 nanometers in one aspect, at least 1000 nanometers in another aspect, or at least 1500 nanometers in another aspect. Optical devices 40 and 100 also can similarly provide a broad bandwidth of circularly-polarized light.

This circularly-polarized light 34 _(c) can be absorbed at or emitted from the distal-end 23 _(exitD) of the exit-rhomb 23 _(exit) into an optical system 31. The optical system 31 can be a beam dump for removing light, a sensor, or other optical device, such as a device that utilizes circularly-polarized light.

At least a portion of the circularly-polarized light 34 _(c) can be emitted from or reflected back from the optical system 31 into the distal-end 23 _(exitD) of the exit rhomb 23 _(exit), and can be linearly polarized in the exit-rhomb 23 _(exit) to a second-polarization 34 _(s) (e.g. s-polarized light), the second-polarization 34 _(s) being opposite of the first-polarization 34 _(p). This light with the second-polarization 34 _(s) from the exit-rhomb 23 _(exit) can pass through the exit-face 13 _(exit), into of the second prism 16, and to the polarizer 24, where it can reflect off of the polarizer 24 towards and at least partially absorbed at and/or transmitted at least partially through the elimination-face 13 _(elimP).

A second-polarization of light 34 _(s) (e.g. s-polarized light), from the light source 37, can reflect off of the polarizer 24 and can be at least partially absorbed at and/or transmitted at least partially through the reflection-face 13 _(refl). The optical device 20 can include another Fresnel rhomb 23, called a reflection-rhomb 23 _(refl), as described above, attached to the reflection-face 13 _(refl). The second-polarization of light 34 _(s), if transmitted through the reflection-face 13 _(refl) can transmit into the proximal-end 23 _(reflP) of the reflection-rhomb 23 _(refl) where it can be circularly polarized as it passes through the reflection-rhomb 23 _(refl). The circularly-polarized light 34 _(c) can be absorbed at or enter a second optical system 32. The second optical system 32 can be located to face and receive light from the distal-end 23 _(reflD) of the reflection-rhomb 23 _(refl).

The second optical system 32 can be a device that utilizes circularly-polarized light. The second optical system 32 can be or can include a sensor for analyzing the circularly-polarized light 34 _(c) from the reflection-rhomb 23 _(refl). The sensor can provide corrective feedback for the light source 37 based on this analysis. For example, the sensor can indicate the amount of light it receives, which indicates the amount of the second-polarization of light 34 _(s) emitted from the light source 37. Alternatively, the second optical system 32 can be or can include a beam dump for removing light from the overall optical system.

At least a portion of the circularly-polarized light 34 _(c) can be emitted from or reflected back from the second optical system 32 and into and through the reflection-rhomb 23 _(refl), where it can be linearly polarized to the first polarization 34 _(p) (e.g. p-polarized light) and emitted from the proximal-end 23 _(reflP) of the reflection-rhomb 23 _(refl) to and through the polarizer 24 then to and at least partially absorbed at and/or at least partially transmitted through the elimination-face 13 _(elim).

The optical device 20 can include another Fresnel rhomb 23, called an elimination-rhomb 23 _(elim), as described above, attached to the elimination-face 13 _(elim). The second-polarization of light 34 _(s) reflected off of the polarizer 24 from the exit-rhomb 23 _(exit) and/or the first polarization 34 _(p) of light emitted through the polarizer 24 from the reflection-rhomb 23 _(refl) can enter the proximal-end 23 _(elimP) of the elimination-rhomb 23 _(elim) where it can be circularly polarized as it passes through the elimination-rhomb 23 _(elim). The circularly-polarized light 34 _(c) can be absorbed at or enter a third optical system 33. The third optical system 33 can be located to face and receive light from the distal-end 23 _(elimD) of the elimination-rhomb 23 _(refl).

The third optical system 33 can be a device that utilizes circularly-polarized light. The third optical system 33 can be or can include a sensor for analyzing the circularly-polarized light 34 _(c) from the elimination-rhomb 23 _(elim). The sensor can provide corrective feedback for the light source 37 based on this analysis. Alternatively, the third optical system 33 can be or can include a beam dump for removing light from the overall optical system.

Dimensions and Angles

As shown in FIGS. 4-6, each of the Fresnel rhombs 23 can have four sides 23 _(sac), 23 _(so), 23 _(sadj), and 23 _(sad j), extending between two ends 23 _(P) and 23 _(D). The four sides 23 _(sac), 23 _(so), 23 _(sadj), and 23 _(sadj) can include an acute-angle-side 23 _(sac), an opposite-side 23 _(so) that is opposite of the acute-angle-side 23 _(sac), and two adjacent-sides 23 _(sadj) that are adjacent to and extend between the acute-angle-side 23 _(sac) and the opposite-side 23 _(sc).

Each Fresnel rhomb 23 can have (1) an acute-angle A_(c) which is an internal angle between the acute-angle-side 23 _(sac) and the outer face 13 to which it is attached; (2) an adjacent-angle A_(adj) which is an internal angle between one or both of the two adjacent-sides 23 _(adj) of each of the Fresnel rhombs 23 and the outer-face of the prism 15 or 16 to which it is attached; and (3) an angle-of-rotation A_(r) between a plane 41 parallel to at least one of the ends of the prisms 15 or 16 and an edge 42 of the Fresnel rhomb 23 where the Fresnel rhomb 23 attaches to the outer face 13.

Each Fresnel rhomb 23 can have (1) a width W_(ac) between the acute-angle-side 23 _(sac) and the opposite-side 23 _(so); (2) a width W_(adj) between two adjacent-sides 23 _(sadj); and (3) a length L between the ends 23 _(P) and 23 _(D). The widths W_(ac) and W_(adj) are measured at an angle parallel to the outer-face 13 to which the Fresnel rhomb 23 is attached. The length L is measured perpendicular to the outer-face 13 to which the Fresnel rhomb 23 is attached.

The acute-angle A_(c), the adjacent-angle A_(adj), the angle-of-rotation A_(r), widths W_(ac) and W_(adj), and the length L can be configured to produce circularly-polarized light. Details of these angles and dimensions will be described in the following few paragraphs. Information related to these dimensions and angles is taught in R. M. A. Azzam, “Relations between amplitude reflectances and phase shifts of the p and s polarizations when electromagnetic radiation strikes interfaces between transparent media,” Appl. Opt. 18, 1884-1886 (1979), which is incorporated herein by reference.

Snell's law states that light which encounters the interface of two mediums will be refracted according to the equation:

n₁ sin θ_(i)=n₂ sin θ_(t);

where n₁=index of refraction of the first medium, θ_(i)=Angle of Incidence of the light at the interface between the two mediums, n₂=index of refraction of the second medium, and θ_(t)=transmitted angle through the second medium. Both angles θ_(i) and θ_(t) are measured with respect to a line perpendicular to a plane of contact between the two mediums.

For example, FIG. 7 shows an incident light beam 73 transmitted through a first medium (e.g. glass with n₁=1.5) into the second medium (e.g. air with n₂=1). The incident light beam 73 has an Angle of Incidence θ_(i)=30°. According to Snell's law, the transmitted light beam 74 has a transmitted angle θ_(t)=48.6°. The Angle of Incidence θ_(i) and the transmitted angle θ_(t) are measured with regard to a line 71 perpendicular to a plane of contact 72 between first medium and second medium.

If n₁>n₂ (e.g. glass is the first medium with n₁=1.5 and air is the second medium with n₂=1), there exists an angle of incidence θ_(i) above which all incident light is reflected, called the critical angle θ_(c). The critical angle (θ_(i)=θ_(c)) occurs when θ_(t)=90° and sin θ_(t)=1. Rearranging Snell's law with the new values:

$\theta_{c} = {\sin^{- 1}\left( \frac{n_{2}}{n_{1}} \right)}$

For example, FIG. 8 shows an incident light beam 73 with an Angle of Incidence θ_(i)=41.8° that is equal to the critical angle θ_(c). According to Snell's law, the transmitted light 74 has a transmitted angle θ_(t)=90°, i.e. along the plane of contact 72 between the two mediums. FIG. 9 shows an incident light beam 73 with an Angle of Incidence θ_(i)=45°, which is greater than the critical angle θ_(c) (41.8°). Therefore, there is no transmitted light beam 74, but instead there is a reflected light beam 94 with an angle of reflection θ_(r) that is equal to the Angle of Incidence θ_(i)=45°.

For light transmitting from N-BK7 glass (the first medium) to air (the second medium), the critical angle θ_(c) for different wavelengths is shown in the following table:

Wavelength Index of Refraction Critical Angle θ_(c) (nm) (n1) (degrees) 300 1.5528 40.1 400 1.5308 40.8 500 1.5214 41.1 600 1.5163 41.3 700 1.5131 41.4 800 1.5108 41.4 1000 1.5075 41.6 Light, in the wavelength range of 300-1000 nanometers (nm), transmitted through N-BK7 glass (first medium) towards air (second medium), will reflect back into the N-BK7 glass as long as the Angle of Incidence θ_(i) is greater than 41.6 degrees (the largest critical angle θ_(c) of all wavelengths in the range).

The function of a Fresnel rhomb 23 is to take linearly polarized light and convert it into circularly polarized light. For circularly polarized light:

-   -   1. The linearly-polarized components (s and p) have the same         amplitude; and     -   2. The linearly-polarized components (s and p) have a 90° phase         shift with regard to each other.         Regarding item #1 for circularly polarized light, proper         angle-of-rotation A_(r) and adjacent-angle A_(adj) (both defined         above and shown in FIGS. 4 and 6 respectively) can result in the         linearly-polarized components (s and p) having the same         amplitude.

Typically, in a polarizing-cube, wires of the polarizer 24 (if the polarizer 24 is a wire grid polarizer) extend between and perpendicular to the ends 11 of the prisms 15 and 16 (i.e. into the page of FIG. 3). With wires of the polarizer 24 extending in the direction just noted, ideal angle-of-rotation A_(r) is 45 degrees. Examples of acceptable angle-of-rotation A_(r) include at least 43 degrees in one aspect, at least 44 degrees in another aspect, or at least 44.5 degrees in another aspect, and less than 45.5 degrees in one aspect, less than 46 degrees in another aspect, or less than 47 degrees in another aspect. If the wires of the polarizer 24 extend in a 45 degree diagonal across the inner face 12 of the prisms, then the ideal angle-of-rotation A_(r) can be 0 degrees.

Ideal adjacent-angle A_(adj) is 90 degrees, but examples of acceptable adjacent-angles A_(adj) include at least 88 degrees in one aspect, at least 89 degrees in another aspect, or at least 89.5 degrees in another aspect, and less than 90.5 degrees in one aspect, less than 91 degrees in another aspect, or less than 92 degrees in another aspect.

Regarding item #2 for circularly polarized light, a total phase shift difference of 90° can be achieved by two internal reflections that each induce a phase shift of 45°. For total internal reflection (TIR), the phase shift between the two polarization states is governed by:

${\tan \frac{\Delta}{2}} = \frac{\left( {{N^{2}\left( {\sin \; \theta_{i}} \right)}^{2} - 1} \right)^{1/2}}{N\; \sin \; \theta_{i}\tan \; \theta_{i}}$

where Δ=phase shift, N=n1/n2, and θ_(i)=Angle of Incidence.

For N-BK7 glass surrounded by air, and 300 nm incident light, the graph in FIG. 10 illustrates the phase shift experienced during TIR.

The Fresnel rhomb can produce circularly polarized light with two phase shifts of 45°, which occurs either at an Angle of Incidence of 45.1° or 57.2°. The below table shows the two Angles of Incidence AOI 1 and AOI 2 at which this 45° phase delay occurs for different wavelengths of light in N-BK7 glass surrounded by air.

Index of Wavelength Refraction (nm) (n1) AOI 1 AOI 2 300 1.5528 45.1 57.2 400 1.5308 46.3 56.3 500 1.5214 47.3 55.7 600 1.5163 47.8 55.2 700 1.5131 48.3 55 800 1.5108 48.5 54.7 900 1.509 48.7 54.6 1000 1.5075 48.9 54.3

The widths W_(ac) and W_(adj) and the length L can be adjusted to ensure that there are two reflections of the light prior to exiting the Fresnel rhomb 23. Width W_(ac) can equal width W_(adj). Following are example materials, dimensions, and angles of a Fresnel rhomb 23 to circularly-polarize light:

-   1. Fresnel rhomb material: N-BK7 -   2. Surrounding medium: air -   3. Light wavelength range 400-1500 nm -   4. Acute-angle A_(c)=55.3° -   5. Adjacent-angle A_(adj)=90° -   6. Angle-of-rotation A_(r)=45° -   7. Width W_(ac)=10 mm -   8. Width W_(adj)=10 mm -   9. Length L=19.166 mm

In order to circularly polarize a large amount of light entering the optical device 20 or 40, it can be important for each Fresnel rhomb 23 to cover a large percent of the outer face 13 to which each is attached. For example, the Fresnel rhomb(s) 23 can cover at least 35% in one aspect, at least 40% in another aspect, at least 45% in another aspect, or at least 50% in another aspect, of the outer face 13 to which it is attached.

Optical device 100 in FIG. 11 can be particularly useful for optical isolation. Light 34 can pass through an aperture 101 _(a) of a light blocking device 101. A light source 37 (shown in FIG. 3) and the aperture 101 _(a) can be positioned for non-normal incident light on the entry-face 13 _(entry) of the first prism 15. By proper selection of the size of the aperture 101 _(a), distance of the light blocking device 101 from the entry-face 13 _(entry), and angle of incidence of the light 34, reflected light 34 _(R) from the entry-face 13 _(entry) can be blocked and also possibly absorbed by the light blocking device 101.

It can be important, for circular polarization of light 34 in the exit-rhomb 23 _(exit), for the light 34 to exit perpendicularly through the exit-face 13 _(exit) of the second prism 16. Therefore, because light incident on the entry-face 13 _(entry) is not normal to it, the entry-face 13 _(entry) and the exit-face 13 _(exit) in this embodiment won't be parallel. For example, an angle θ1 between a plane 102 of the entry-face 13 _(entry) of the first prism 15 and a plane 103 of the exit-face 13 _(exit) of the second prism can be: at least 3° in one aspect, at least 6° in another aspect, between 4.5° and 13° in another aspect, or between 6° and 11° in another aspect.

It can be important, for circular polarization of light 34 in the exit-rhomb 23 _(exit), for the entry-face 13 _(entry) of the first prism 15 and the distal-end 23 _(exitD) of the exit-rhomb 23 _(exit) to be parallel or very close to parallel. Therefore an angle θ2 between a plane 102 of the entry-face 13 _(entry) of the first prism 15 and a plane 104 of the distal-end 23 _(exitD) of the exit-rhomb 23 _(exit) can be <2° in one aspect, <1° in another aspect, <0.5° in another aspect, or <0.1° in another aspect.

It can be helpful in some applications for an angle θ3 between a plane 103 of the exit-face 13 _(exit) of the second prism 16 and a plane 105 of the polarizer 24 to be equal or close to 45°, such as for example in the range of 45° +/− 1.5° in one aspect or 45° +/− 5° in another aspect. 

What is claimed is:
 1. An optical device comprising: a polarizing-cube including a polarizer and a pair of prisms defining a first prism and a second prism, each prism having two ends linked by an inner-face and two outer-faces, the prisms are attached together at the inner-face of each with the polarizer sandwiched between the prisms; one outer-face of the first prism defines an entry-face and the other outer-face of the first prism defines a reflection-face; one outer-face of the second prism, located opposite of the entry-face of the first prism, defines an exit-face, and the other outer-face of the second prism, located opposite of the reflection-face of the first prism defines an elimination-face; a Fresnel rhomb, defining an exit-rhomb, having a proximal-end attached to the exit-face of the second prism and having a distal-end spaced apart from the exit-face and opposite of the proximal-end; and an angle (θ1) between a plane of the entry-face of the first prism and a plane of the exit-face of the second prism is at least 3°.
 2. The optical device of claim 1, wherein the exit-rhomb is attached to the second prism by an adhesive-free, optical-bond.
 3. The optical device of claim 1, wherein the pair of prisms and the exit-rhomb are made of the same material.
 4. The optical device of claim 1, wherein an angle-of-rotation of the exit-rhomb is between 44 degrees and 46 degrees, where the angle-of-rotation is an angle between a plane parallel to at least one of the ends of the prisms and an edge of the exit-rhomb at the exit-face.
 5. The optical device of claim 1, wherein: the exit-rhomb has four sides extending between the proximal-end and the distal-end, the four sides including: an acute-angle-side having an acute-angle that is an internal angle between the acute-angle-side and the exit-face of the second prism; an opposite-side that is opposite of the acute-angle-side; and two adjacent-sides that are adjacent to and extend between the acute-angle-side and the opposite-side; and the acute-angle of the exit-rhomb, and a length of the exit-rhomb between the proximal-end and the distal-end, are configured for two internal reflections of a wavelength of light in the wavelength range of intended use.
 6. The optical isolator of claim 5, wherein an internal angle, between each of the two adjacent-sides of the exit-rhomb and the exit-face of the second prism is between 89 degrees and 91 degrees.
 7. The optical isolator of claim 1, wherein the exit-rhomb has a length for circular polarization of light.
 8. The optical isolator of claim 1, wherein the exit-rhomb covers at least 35% of the exit-face of the second prism.
 9. The optical device of claim 1, further comprising another Fresnel rhomb, defining an elimination-rhomb, having a proximal-end attached to the elimination-face of the second prism and a distal-end spaced apart from the elimination-face and opposite of the proximal-end.
 10. The optical device of claim 1, further comprising another Fresnel rhomb, defining a reflection-rhomb, having a proximal-end attached to the reflection-face of the first prism and a distal-end spaced apart from the reflection-face and opposite of the proximal-end.
 11. The optical device of claim 1, wherein θ1 is at least 6°.
 12. The optical device of claim 1, wherein θ1 is between 4.5° and 13°.
 13. The optical device of claim 1, wherein θ1 is between 6° and 11°.
 14. The optical device of claim 1, wherein an angle between the entry-face of the first prism and a plane of the distal-end of the Fresnel rhomb is <2°.
 15. The optical device of claim 1, wherein an angle between the entry-face of the first prism and a plane of the distal-end of the Fresnel rhomb is <1°.
 16. The optical device of claim 1, wherein an angle between the entry-face of the first prism and a plane of the distal-end of the Fresnel rhomb is <0.5°.
 17. The optical device of claim 1, wherein an angle between the entry-face of the first prism and a plane of the distal-end of the Fresnel rhomb is <0.1°.
 18. An optical device comprising: a polarizing-cube including a polarizer and a pair of prisms defining a first prism and a second prism, each prism having two ends linked by an inner-face and two outer-faces, the prisms are attached together at the inner-face of each with the polarizer sandwiched between the prisms; one outer-face of the first prism defines an entry-face and the other outer-face of the first prism defines a reflection-face; one outer-face of the second prism, located opposite of the entry-face of the first prism, defines an exit-face, and the other outer-face of the second prism, located opposite of the reflection-face of the first prism defines an elimination-face; a Fresnel rhomb, defining an exit-rhomb, having a proximal-end attached to the exit-face of the second prism and having a distal-end spaced apart from the exit-face and opposite of the proximal-end; a Fresnel rhomb, defining an elimination-rhomb, having a proximal-end attached to the elimination-face of the second prism and a distal-end spaced apart from the elimination-face and opposite of the proximal-end; the Fresnel rhombs are attached to the second prism by an adhesive-free, optical-bond; and an angle (θ1) between a plane of the entry-face of the first prism and a plane of the exit-face of the second prism is at least 3°.
 19. An optical device comprising: a polarizing-cube including a polarizer and a pair of prisms defining a first prism and a second prism, each prism having two ends linked by an inner-face and two outer-faces, the prisms are attached together at the inner-face of each with the polarizer sandwiched between the prisms; one outer-face of the first prism defines an entry-face and the other outer-face of the first prism defines a reflection-face; one outer-face of the second prism, located opposite of the entry-face of the first prism, defines an exit-face, and the other outer-face of the second prism, located opposite of the reflection-face of the first prism defines an elimination-face; a Fresnel rhomb, defining an exit-rhomb, having a proximal-end attached to the exit-face of the second prism and having a distal-end spaced apart from the exit-face and opposite of the proximal-end; a Fresnel rhomb, defining a reflection-rhomb, having a proximal-end attached to the reflection-face of the first prism and a distal-end spaced apart from the reflection-face and opposite of the proximal-end; the Fresnel rhombs are attached to the second prism by an adhesive-free, optical-bond; and an angle (θ1) between a plane of the entry-face of the first prism and a plane of the exit-face of the second prism is at least 3°.
 20. The optical device of claim 19, wherein the pair of prisms, the exit-rhomb, and the reflection-rhomb are made of the same material. 